1. Field of the Invention
The present invent relates to instrumented sensors and to apparatus and methods for measuring or predicting stress and/or stress component(s) at an interface, such as a bond line, a joint, etc. of mated bodies.
2. Description of the Related Art
Various conventional means exist for joining two mechanical bodies together. By way of example, one or more pin members may be disposed through aligned and mating apertures of the mechanical bodies. Another example of a means for joining mechanical bodies together is adhesive, which may be applied along an adhesive bond line at the interface of the two mated bodies. Alternatively, adhesives may be used in combination with mechanical connectors, such as pin members.
Under operating conditions, a variety of forces act on the interface. For example, in the case of a bond line, these forces include normal forces acting perpendicular to a bond line and shear forces acting along the bond line.
Apparatuses and methods have been known for instrumenting a joint means to measure normal and shear stresses. For example, the use of various types of strain gauges to measure such forces are well known. Sensing devices are also known in which such strain gauges are used in combination with bridge circuitry such as a Wheatstone bridge.
The accurate measurement of shear forces exclusive of normal forces, however, has eluded effective measurement. Shear forces may be particularly insidious forces, especially along an adhesive bond line. It is often highly desirable to design a component or system so that shear forces and their detrimental effects can be minimized. The accurate measurement of shear forces often is particularly difficult, however, because the magnitude of the force can change, sometimes rapidly, over time and because shear forces are often accompanied by normal forces.
In the case of solid rocket motors, for example, there is an adhesive interface or bond line between the solid propellant grain and the insulated casing member. This adhesive interface is commonly referred to as a liner and is often made of polyurethane adhesives and the like. The liner functions to provide the bond between the propellant and casing insulation with adequate adhesive strength to ensure that the interfacial bond will be capable of withstanding all of the stresses to which the propulsion subsystem may be subjected during ignition, launch, maneuver, etc. The shear forces present during rocket motor operation, especially at launch, place great stress on the liner. Failure of the adhesive bond at the liner can lead to cracking or premature discharge of the solid propellant, thus compromising the rocket motor operation.
Preparation of adequate liner compositions and structures requires accurate modeling of shear loads experienced at the propellant-insulation interface. However, the accurate measurement of the shear loads in this environment has been difficult. Conventional shear sensors are sensitive to normal loads, temperature changes and other varying conditions experienced during rocket motor operation. These outside influences can lead to inaccurate shear stress measurements.
Accordingly, the present invention according to one aspect provides an instrumented sensor that can effectively measure stress or a stress component expected at an interface, especially an adhesive bond line or joint, of mated bodies. Another feature of this aspect is the measurement of shear stress substantially exclusive of stress normal to the interface.
The present invention according to another aspect provides a system comprising two mated bodies and an instrumented sensor that can effectively measure stress and a stress component expected at an interface, especially an adhesive bond line or a joint, of the mated bodies. Another feature of this aspect is the measurement of shear stress substantially exclusive of stress normal to the interface.
The present invention according to still another aspect provides a method for measuring stress and a stress component expected at an interface, especially an adhesive bond line or a joint, of mated bodies, particularly for measuring shear stress substantially exclusive of stress normal to the interface.
Additional advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations pointed out in the appended claims.
In accordance with the purposes of the invention as embodied and broadly described in this document, a stress sensor is provided. The sensor comprises a sensor body (or sensor housing), a sensing device, and, optionally, a sensor measurement signal output device. The sensor body comprises a first wall and a second wall coupled to one another, the first wall and second wall each having a respective portion (xe2x80x9copposing portionsxe2x80x9d) opposing one another. The opposing portion of the first wall and the opposing portion of the second wall extend parallel to one another and are spaced apart from one another in a direction along a y-axis that is perpendicular to a central x-axis. The central x-axis extends parallel to and equidistant from the opposing portions of the first and second walls. The sensor body is resiliently deformable in response to a physical stress having a shear component and, optionally, a normal component. The shear component of the physical stress causes deformation of the sensor body by moving the first wall relative to the second wall along a direction generally parallel to the x-axis. The optional normal component of the physical stress causes deformation of the sensor body by moving the first wall relative to the second wall along a direction parallel to the y-axis. The sensing device comprises first and second sensor elements, each extending between the opposing portions of the first and second walls for outputting sensor measurement signals representative of the physical stress. The first sensor element has a first longitudinal axis intersecting the central axis at a first oblique angle xcex1 and the second sensor element has a second longitudinal axis intersecting the central axis by a second oblique angle xe2x88x92xcex1. The arrangement of the first and second sensor elements permits measurement of the stress and, more preferably, a component (or components) of the stress. In a particular embodiment, from the sensor measurement signals, the shear component preferably can be determined substantially exclusive of the normal component. The sensor measurement signal output device outputs the sensor measurement signals from the sensor device.
The sensor body may comprise a metal or alloy, such as aluminum or aluminum alloys. Preferably, the sensor body consists essential of aluminum.
The sensor body preferably comprises third and fourth walls spaced apart from and opposing one another and each extending in a respective plane parallel to the y-axis, the third and fourth walls coupling the first and second walls to one another to provide a block with a quadrangular cross section (when in a nondeformed state). In one embodiment, the quadrangular cross section is rectangular. In another embodiment, the quadrangular cross section is rectangular and has a length-to-height ratio of about 4 to 1. In still another embodiment, the quadrangular cross section is square. The sensor body may have an open chamber with a periphery bounded by the first, second, third, and fourth walls.
In another embodiment, the sensor body comprises third and fourth parallel walls spaced apart from and opposing one another, and fifth and sixth parallel walls spaced apart from and opposing one another. In this embodiment, the six walls form a block, preferably having an enclosed chamber bounded by six walls.
In another embodiment, the sensor body comprises at least first, second, third, and fourth walls forming a block with first and second pairs of diagonally opposed corners. The first sensor element comprises a first strain gauge having opposite ends respectively connected proximate to the first pair of diagonally opposed corners of the block to extend diagonally across the block. Likewise, the second sensor element comprises a second strain gauge having opposite ends respectively connected proximate to the second pair of diagonally opposed corners of the block to extend diagonally across the block and cross the first sensor element.
In the exemplary embodiments, the first and second sensor elements are strain gauges, each having a respective longitudinal axis. The first and second strain gauges are each arranged to undergo equal compression or extension along the longitudinal axes thereof representative of the normal component of the deformation stress applied to the sensor body. Also, the first strain gauge is arranged to undergo compression along the longitudinal axis thereof and the second strain gauge is arranged to undergo extension along the longitudinal axis thereof of equal magnitude (yet in opposite directions) representative of the shear component of the physical stress.
It is preferred that the first and second sensor elements each comprise a respective optical strain gauge, which are preferably symmetrical to each other across the x-axis. Preferred optical strain gauges undergo a corresponding deformation in response to a physical parameter to alter the optical characteristic of light signals being reflected therein or transmitted therethrough. The optical characteristics altered by the physical parameter may be selected from the group consisting of light intensity, phase, wavelength, and the like.
Where the sensor elements comprise optical fiber strain gauges, the sensor measurement signal output device preferably comprises an optical-to-electrical converter.
In accordance with another aspect of the invention, a system is provided for measuring stress at an interface, such as a bond line or a joint, between the first and second mated bodies. The system comprises the first and second mated bodies, a stress sensor situated at the interface, and a data-receiving device. Suitable stress sensors for the system of this aspect of the invention include, not necessarily by limitation, any and all of the stress sensors described or illustrated herein, either singularly or in plural. The sensor includes a sensor body having a first wall coupled to the first mated body and a second wall coupled to the second mated body, a sensing device, and a sensor measurement signal output device. The data-receiving device is operatively coupled to the sensor measurement output device for receiving sensor output signals.
Preferably, but optionally, the system comprises a plurality of the stress sensors. Also preferably but optionally, the data-receiving device comprises at least one of a data processor and a data display.
The system of this aspect of the invention is useful in the context of measuring stresses imparted by physical loads in a rocket motor. For example, the first body may comprise a casing member or insulation layer of a rocket motor and the second body may comprise a solid propellant grain of the rocket motor. In this system, it is especially desirable to embed the sensor in the liner situated between the solid propellant grain and the insulated casing member.
In accordance with yet another aspect of the invention, a method is provided for measuring shear stress at an interface between first and second mated bodies. The method comprises disposing a stress sensor at the interface, such as an adhesive bond line or joint, between the first and second mated bodies. Suitable stress sensors for the method of this aspect of the invention include, but are not necessarily limited to, any and all of the stress sensors described or illustrated herein, either singularly or in plural. The sensor includes a sensor body, a sensing device, and a sensor measurement signal output device. A first wall of the sensor body is coupled to the first mated body and a second wall of the sensor body is coupled to the second mated body. The method further comprises sensing deformation stress applied to the sensor body and outputting sensor measurement signals representative of the deformation stress. In an especially preferred embodiment of this aspect of the invention, the sensor measurement signals are communicated to a data-receiving device, and the shear component of the deformation stress is capable of being determined substantially exclusive, and more preferably completely exclusive, of the normal component of the deformation stress.
Preferably, but optionally, the method comprises using a plurality of the stress sensors. Also preferably but optionally, the data-receiving device comprises at least one of a data processor and a data display.
The method of this aspect of the invention is useful for measuring stresses imparted by physical loads in a rocket motor, such as during launch of the rocket motor. For example, the first body may comprise a casing member or, more likely, an insulation layer of a rocket motor and the second body may comprise a solid propellant grain of the rocket motor. In this method, it is especially desirable to embed the sensor in the liner situated between the solid propellant grain and the insulated casing member.